The invention relates to a clocked power end stage for inductive loads, including a free-wheeling circuit, a series circuit being located at poles of a voltage source and having the load and a switch device, in particular a bipolar or MOS transistor, with a continuous transition between ON and OFF states, and a control circuit for generating a control clock signal for the switch device.
In order to control inductive loads, clocked power end stages with transistors, especially MOSFETs, are often used, because they have a lower power loss. A disadvantage of such clocked end stages is that they cause considerable electromagnetic interference.
One known cause of the electromagnetic interference is a rapid change in voltage brought about by the clocking, at a connecting point between the load and the switch device. Such a cause of electromagnetic interference is encountered, in circuits with MOSFETs for instance, by high-impedance triggering of the control terminal by means of a resistor.
Another cause of the electromagnetic interference that is brought about is a rapid change in the current flowing through the switch device, which is effected by the clocking. The speed of the change in current depends on the steepness of a free-wheeling or recovery diode, that is typically connected parallel to the inductive load, and on the aforementioned speed of the voltage change at the connecting point between the load and the switch device.
While it is virtually impossible to vary the steepness of the diode, the speed of the voltage change can be varied, as already noted, by means of high-impedance triggering of the switch device. However, in order to effect the necessary lessening in the speed of the current change, the speed of the voltage change would have to be reduced substantially more markedly than would be necessary for reducing the electromagnetic interference caused by the voltage change itself. However, in that case, the power loss arising in the switch device would be increased to such an extent that the advantage attained by the clocked triggering would be lost after all.
It is accordingly an object of the invention to provide a clocked power end stage for inductive loads, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and to do so in such a way that the electromagnetic interference brought about by it is reduced further, yet without substantially increasing the power loss.